An aluminum alloy is lightweight and has high thermal conductivity, and also achieves high corrosion resistance when properly treated. Thus, aluminum alloys are used for heat exchangers for automotive and other applications, such as radiators, condensers, evaporators, heaters, intercoolers, and oil coolers. Conventional tube materials for automotive heat exchangers include, for example, a two-layer clad material and a three-layer clad material, where the two-layer clad material has a core material made of an Al—Mn based alloy, such as 3003 alloy, and a brazing filler metal made of an Al—Si based alloy or a sacrificial anode material made of an Al—Zn based alloy that is clad on one surface of the core material, while the three-layer clad material additionally has a brazing filler metal made of an Al—Si alloy that is clad on the other surface of the core material. For a heat exchanger, in general, such clad material and a corrugated fin material are combined and brazed at a temperature as high as about 600° C. to be joined together.
Oil coolers, for example, usually employ a water cooling system in which heat is exchanged between engine oil and cooling water to cool the engine oil. Nowadays some intercoolers also employ such water cooling system. As the cooling water, an LLC containing an anti-corrosive agent as an additive should be used, but tap water or well water may sometimes be used in developing countries, for example. The tap water or well water can contain chloride ions, and thus may destroy a film of aluminum oxide to cause pitting corrosion, resulting in corrosion perforation in the passage for cooling water.
A typical countermeasure against this problem is cladding a sacrificial anode material made of an Al—Zn based alloy to give the sacrificial protection feature, so as to prevent the corrosion perforation caused by pitting corrosion. One method for forming a passage for cooling water in a heat exchanger includes stacking plates 1, which are made by forming a clad material into a passage for cooling water, via corrugated fins 2 as illustrated in FIG. 1. This method is advantageous because a heat exchanger can be changed in size merely by changing the number of stacks, thus providing higher design flexibility. However, to join plates together, the plate material itself is needed to supply a brazing filler metal during brazing. In addition, when Zn is used to give the aforementioned sacrificial protection feature, Zn becomes concentrated in the brazing filler metal that has gathered on a joint portion. This results in the problem that the pitting potential in the joint portion becomes significantly less noble to cause preferential corrosion in the joint portion.
In light of the foregoing, in order to apply the stacked-plate type to a water-cooling heat exchanger as seen in FIG. 1, it is necessary to clad layers that have a plurality of functions including: supplying a brazing filler metal during brazing to the inner side of the passage made of a material used for the passage forming component; having a sacrificial protection feature against pitting corrosion; and preventing preferential corrosion in the joint portion.
Techniques for supplying a brazing filler metal during brazing and giving a sacrificial protection feature against pitting corrosion are described in Patent Literatures 1 and 2. These patent literatures state that an Al—Zn based intermediate layer having the sacrificial protection feature is disposed between the brazing filler metal and the core material, with the result that both the brazability and the sacrificial protection feature can be accomplished. However, these patent literatures neither recognize the problem of preferential corrosion in a joint portion nor describe any method for preventing such problem.